This invention relates to microwave windows, and more particularly to microwave windows adapted for use in a low-pressure (i.e., vacuum) chamber to enable the introduction of high-power microwave energy from a source external to the chamber to pass through such window into such chamber.
As is known in the art, many applications require the introduction of high-power microwave energy into a low-pressure chamber from a microwave source external to the chamber. One such application is plasma enhanced chemical vapor deposition (PECVD) processing. In such application, the microwave energy is introduced into the chamber (or reactor) through a dielectric window. Thus, the window is disposed on an open portion of the wall of the chamber. A waveguide terminates on an outer surface of the window. Because the gas pressure in the chamber (or reactor) is low and the microwave electric field (i.e., the E-field) is at its highest level at the reactor, there is a problem of Paschen breakdown on the window. When this occurs, high-density plasma forms on the reactor side (i.e., low pressure or vacuum-side) of the window. This plasma strongly absorbs the incoming microwave energy leading to high localized heating of the window. Under certain conditions, this heating causes failure of the window due to thermal shock. Window failure causes the reactor to vent air thereby destroying the high vacuum within the chamber and the desired processing within the chamber.
In accordance with the present invention, a microwave window structure for a low-pressure chamber is provided. The window structure enables microwave energy to be introduced into the chamber from a source external to the chamber. The window structure includes a fixture having electrically conductive walls. Inner portions of the walls provide a peripheral region of an aperture within such fixture. The fixture is adapted for mounting to a sidewall portion of the chamber having an opening therethrough. A solid, microwave energy transparent dielectric window is included in the window structure. The window includes: a periphery portion affixed to the fixture; and, an inner region disposed with the aperture of the fixture and aligned with the opening through the chamber sidewall portion. A first surface of the inner region is disposed in the chamber through the chamber opening. A second, opposite surface of the inner region widow is disposed external to the chamber. The window has a sidewall portion with a first end thereof terminating in the first surface of the window and a second end thereof terminating at the periphery portion of the window. The sidewall portion of the window is spaced from the walls of the conductive fixture.
With such an arrangement, the junction at the contact between conductive wall of the fixture, the dielectric window, and the vacuum within the chamber (a so-called xe2x80x9ctriple junctionxe2x80x9d) is displaced from the conductive walls of the fixture. More particularly, one key phenomenon in initiating microwave breakdown across a surface is injection of electrons onto the dielectric surface from the triple junction. Here, electrons are field-emitted from the conductive surface, especially from sharp edges or burrs in the walls. Each collision between the emitted electrons with the surface of the dielectric window multiplies the number of electrons because the secondary yield of most dielectrics is greater than unity (for example, up to a multiplier of 4 for silica). As the electrons cross the surface of the dielectric window they produce avalanche, causing ionization and gas breakdown to occur on the window.
In one embodiment, the periphery portion of the window contacts the inner portions of the walls of the fixture adjacent a rounded region of such inner portions of the walls. With such an arrangement, the rounded region reduces the amplitude of the electric field at the triple junction therefore the number of field emitted electrons is correspondingly reduced.
In one embodiment, the sidewall portion of the window is parallel to the inner walls of the fixture. With such an arrangement, the direction of the electric field established across opposing portions of the conductive wall is normal to the surface of the adjacent portion of the window (i.e., normal to the displaced sidewalls of the window).
In one embodiment, the second surface of the window includes thereto a corrugated structure. With such an arrangement, the magnitude of the electric field is more uniform across the window thereby reducing the peak electric field across the window.
In one embodiment, the first surface of the window has peaks and valleys therein, such peaks being separated one from the other by a length less than the nominal-operating wavelength of the microwave energy being introduced into the chamber through the window. With such an arrangement, the inner surface portion of the window is parallel to the electric field vector is broken up by the subwavelength structures (i.e., the peaks and valleys) to reduce the probability of surface-discharge formation; in addition the same subwavelength structures are designed to eliminate microwave reflections caused by the large differential in dielectric constant between the window and the vacuum.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.